Style leaderboard

Just some basic styling for the leaderboard table. I have made the table
columns' width fixed and added a coloring to the score.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add column to show number of snapshots

Just added an column to display the number of snapshots imported for a
certain repository.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add snapshot selection to the breadcrumb

Snapshots are not selectable through the breadcrumb as well. I went back
to use the date of a snapshot as the actual identifier. This should not
be a problem, since the identifier only needs to be unique with respect
to the parent document.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>